PCRESTACK(3)                         Library Functions Manual                        PCRESTACK(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE DISCUSSION OF STACK USAGE

       When  you  call  pcre[16|32]_exec(),  it makes use of an internal function called match().
       This calls itself recursively at branch points in the pattern, in order  to  remember  the
       state of the match so that it can back up and try a different alternative if the first one
       fails. As matching proceeds deeper and deeper into the tree of possibilities,  the  recur‐
       sion  depth  increases.  The  match()  function is also called in other circumstances, for
       example, whenever a parenthesized sub-pattern is entered, and in certain cases of  repeti‐
       tion.

       Not  all  calls  of match() increase the recursion depth; for an item such as a* it may be
       called several times at the same level, after matching different numbers of a's.  Further‐
       more,  in  a  number  of cases where the result of the recursive call would immediately be
       passed back as the result of the current call (a "tail recursion"), the function  is  just
       restarted instead.

       The above comments apply when pcre[16|32]_exec() is run in its normal interpretive manner.
       If the pattern was studied with the PCRE_STUDY_JIT_COMPILE option, and  just-in-time  com‐
       piling was successful, and the options passed to pcre[16|32]_exec() were not incompatible,
       the matching process uses the JIT-compiled code instead of the match() function.  In  this
       case, the memory requirements are handled entirely differently. See the pcrejit documenta‐
       tion for details.

       The pcre[16|32]_dfa_exec() function operates in an entirely different way, and uses recur‐
       sion  only when there is a regular expression recursion or subroutine call in the pattern.
       This includes the processing of assertion and "once-only" subpatterns, which  are  handled
       like  subroutine calls. Normally, these are never very deep, and the limit on the complex‐
       ity of pcre[16|32]_dfa_exec() is controlled by the amount of workspace it is given.   How‐
       ever,  it  is  possible  to write patterns with runaway infinite recursions; such patterns
       will cause pcre[16|32]_dfa_exec() to run out of stack. At present, there is no  protection
       against this.

       The  comments  that  follow do NOT apply to pcre[16|32]_dfa_exec(); they are relevant only
       for pcre[16|32]_exec() without the JIT optimization.

   Reducing pcre[16|32]_exec()'s stack usage

       Each time that match() is actually called recursively, it uses  memory  from  the  process
       stack.  For  certain kinds of pattern and data, very large amounts of stack may be needed,
       despite the recognition of "tail recursion".  You can often reduce the  amount  of  recur‐
       sion,  and  therefore  the  amount  of  stack used, by modifying the pattern that is being
       matched. Consider, for example, this pattern:

         ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet" or the end of the data, and
       is  the  kind of pattern that might be used when processing an XML file. Each iteration of
       the outer parentheses matches either one character that is not "<" or a "<"  that  is  not
       followed  by "inet". However, each time a parenthesis is processed, a recursion occurs, so
       this formulation uses a stack frame for each matched character. For a long string,  a  lot
       of  stack is required. Consider now this rewritten pattern, which matches exactly the same
       strings:

         ([^<]++|<(?!inet))+

       This uses very much less stack, because runs of characters that do  not  contain  "<"  are
       "swallowed"  in one item inside the parentheses. Recursion happens only when a "<" charac‐
       ter that is not followed by "inet" is encountered (and we assume this is relatively rare).
       A  possessive quantifier is used to stop any backtracking into the runs of non-"<" charac‐
       ters, but that is not related to stack usage.

       This example shows that one way of avoiding stack  problems  when  matching  long  subject
       strings  is  to  write repeated parenthesized subpatterns to match more than one character
       whenever possible.

   Compiling PCRE to use heap instead of stack for pcre[16|32]_exec()

       In environments where stack memory is constrained, you might want to compile PCRE  to  use
       heap  memory  instead  of  stack for remembering back-up points when pcre[16|32]_exec() is
       running. This makes it run a lot more slowly, however.  Details of  how  to  do  this  are
       given  in the pcrebuild documentation. When built in this way, instead of using the stack,
       PCRE obtains and frees memory by  calling  the  functions  that  are  pointed  to  by  the
       pcre[16|32]_stack_malloc  and pcre[16|32]_stack_free variables. By default, these point to
       malloc() and free(), but you can replace the pointers to cause PCRE to use your own  func‐
       tions.  Since  the block sizes are always the same, and are always freed in reverse order,
       it may be possible to implement customized memory handlers that are  more  efficient  than
       the standard functions.

   Limiting pcre[16|32]_exec()'s stack usage

       You can set limits on the number of times that match() is called, both in total and recur‐
       sively. If a limit is exceeded, pcre[16|32]_exec() returns an error code. Setting suitable
       limits  should  prevent it from running out of stack. The default values of the limits are
       very large, and unlikely ever to operate. They can be changed when PCRE is built, and they
       can  also  be  set when pcre[16|32]_exec() is called. For details of these interfaces, see
       the pcrebuild documentation and the section on extra data for  pcre[16|32]_exec()  in  the
       pcreapi documentation.

       As  a  very rough rule of thumb, you should reckon on about 500 bytes per recursion. Thus,
       if you want to limit your stack usage to 8Mb, you should set the  limit  at  16000  recur‐
       sions. A 64Mb stack, on the other hand, can support around 128000 recursions.

       In  Unix-like  environments, the pcretest test program has a command line option (-S) that
       can be used to increase the size of its stack. As long  as  the  stack  is  large  enough,
       another  option  (-M) can be used to find the smallest limits that allow a particular pat‐
       tern to match a given subject string. This is done by calling  pcre[16|32]_exec()  repeat‐
       edly with different limits.

   Obtaining an estimate of stack usage

       The  actual amount of stack used per recursion can vary quite a lot, depending on the com‐
       piler that was used to build PCRE and the optimization or debugging options that were  set
       for it. The rule of thumb value of 500 bytes mentioned above may be larger or smaller than
       what is actually needed. A better approximation can be obtained by running this command:

         pcretest -m -C

       The -C option causes pcretest to output information about the options with which PCRE  was
       compiled.  When  -m  is  also given (before -C), information about stack use is given in a
       line like this:

         Match recursion uses stack: approximate frame size = 640 bytes

       The value is approximate because some recursions need a bit more (up to  perhaps  16  more
       bytes).

       If  the  above command is given when PCRE is compiled to use the heap instead of the stack
       for recursion, the value that is output is the size of each block that  is  obtained  from
       the heap.

   Changing stack size in Unix-like systems

       In  Unix-like  environments,  there is not often a problem with the stack unless very long
       strings are involved, though the default limit on stack size varies from system to system.
       Values  from  8Mb  to 64Mb are common. You can find your default limit by running the com‐
       mand:

         ulimit -s

       Unfortunately, the effect of running out of stack is often  SIGSEGV,  though  sometimes  a
       more explicit error message is given. You can normally increase the limit on stack size by
       code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This reads the current limits (soft and hard) using getrlimit(), then attempts to increase
       the   soft   limit   to   100Mb  using  setrlimit().  You  must  do  this  before  calling
       pcre[16|32]_exec().

   Changing stack size in Mac OS X

       Using setrlimit(), as described above, should also work on Mac OS X. It is  also  possible
       to set a stack size when linking a program. There is a discussion about stack sizes in Mac
       OS X at this web site: http://developer.apple.com/qa/qa2005/qa1419.html.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 24 June 2012
       Copyright (c) 1997-2012 University of Cambridge.



PCRE 8.30                                  24 June 2012                              PCRESTACK(3)